Some of organic light emitting devices are referred to as organic electroluminescence devices. For example, such an organic light emitting device comprises a transparent electrode serving as an anode, a hole transport layer, a light emitting layer (an organic light emitting layer), an electron injection layer and a cathode, which are superimposed in this order and provided on one side of a transparent substrate. The organic electroluminescence device is configured to recombine electrons injected from the electron injection layer with holes injected from the hole transport layer, at the light emitting layer, so as to cause a light emission within the light emitting layer. The device is designed to propagate outwards the light emitted from the light emitting layer, through the transparent electrode and the transparent substrate.
The organic electroluminescence device is designed to give a self-emission light in various wavelengths, with a relatively high yield. Such organic electroluminescence devices are expected to be applied for production of displaying apparatuses, light emitters such as flat panel displays, and light sources such as liquid-crystal displaying backlights and luminaires. Some of organic electroluminescence devices have already been developed for practical uses.
However, the organic electroluminescent device suffers from problems arising from a trade-off relationship between brightness and durability. That is, the organic electroluminescent device may suffer from poor durability, when being configured to give a clear image or radiate a light with a high brightness.
The organic light emitting device has been developed in view of such problems, as disclosed in patent references 1 to 5. Each of the organic light emitting devices contains a plurality of light emitting layers disposed between the anode and the cathode. In this device, the plural light emitting layers are electrically connected to each other.
FIG. 3 illustrates one example of such organic electroluminescence devices. The organic electroluminescence device comprises an anode 1, a cathode 2, an intermediate layer 3, a plurality of light emitting layers 4a,4b and a transparent substrate 5. The light emitting layer 4a,4b are disposed between the anode and the cathode. The intermediate layer 3 is interposed between the light emitting layers 4a,4b. The anode 1, the cathode 2, the intermediate layer 3 and the light emitting layers 4a,4b are laminated to a surface of the transparent substrate 5. The anode 1 is formed of a light transmissive material. The cathode 2 is formed of a light reflective material. The device is provided at both sides of the light emitting layer 4a,4b with an electron injection layer and a hole transport layer, which are not illustrated in FIG. 3.
In this device, the plural light emitting layers 4a,4b are separated from each other with the intermediate layer 3 interposed therebetween, but electrically connected to each other. In response to an electric voltage applied between the anode 1 and the cathode 2, the light emitting layers 4a,4b are stimulated to simultaneously radiate lights just like series-connected light emitting layers, giving a luminous light with a higher brightness by combination of the lights radiated from 4a,4b, compared to conventional organic electroluminescence devices. Besides, this device enables it to overcome the problems arising from the trade-off relationship between brightness and durability mentioned above.
As configurations of the intermediate layer 3, there are generally known configurations, such as (1) BCP:Cs/V2O5, (2) BCP:Cs/NPD:V2O5, (3) a product of reaction between an Li complex and Al, (4) Alq:Li/ITO/hole transport material, (5) a metal-organic mixture layer, (6) an alkali metal oxide or an alkali earth metal oxide. Herein, symbols “:” and “/” denote a mixture of two materials and a laminate of two compositions, respectively.
Other configurations of the intermediate layer 3 have been disclosed in patent reference 6 and non-patent reference 1. The intermediate layers 3 in these references are respectively formed of (7) a laminate including an electron injection layer, an electron extraction layer and an hole transport material layer, and (8) a laminate including an electron injection layer, an electron transport layer, an electron extraction layer and a hole transport layer.